As a method of pyrolyzing a resin, a method of supplying a pyrolysis tank with a resin and high-temperature solid particles and pyrolyzing the resin while fluidizing them with fluidization gas is known. Another method of pyrolyzing a resin in the atmosphere of inert gas such as nitrogen is also known. In these methods, the amount of heat required for pyrolyzing the resin is supplied by the high-temperature solid particles. Accordingly, the methods are advantageous industrially.
As the above-mentioned method of pyrolyzing the resin, the following methods are known.
Patent Document 1 discloses a method of feeding solid waste to a high-temperature fluidized bed, in which solid waste is cooled to prevent thermally-melted materials from being attached thereto by allowing cooling gas between a screw shaft and a screw outer pipe in resin feeding equipment so as to come in direct contact with the solid waste and a rotational force is applied to a fluid medium to destruct bridges of the solid waste and to accomplish a smooth flow by allowing the cooling gas to flow in a fluidized bed in a tangential direction thereof.
However, the method disclosed in Patent Document 1 has a problem with an increase in cost for allowing the cooling gas to flow and a decrease in temperature of the fluidized bed due to the flow of the cooling gas.
Patent Document 2 discloses a method of recovering pyrolysis products, by mixing waste plastic with high-temperature sand by means of mechanical agitation and directly heating the waste plastic to pyrolyze the waste plastic.
In this method, since a pyrolysis tank for pyrolyzing the waste plastic is fluidized by the use of an agitator, there is a problem that the fluidization is not sufficient only by the use of the agitator and long-term stable operation is difficult. Depending on the temperature or the waste plastic condition, there is a problem that the flow in the pyrolysis tank is completely stopped. This method also has a problem in that the pyrolysis products are not smoothly discharged from the pyrolysis tank system and the recovered pyrolysis products decrease in quality. In case of large-sized equipment, a lot of agitating torque is necessary for maintaining the mixture state in the pyrolysis tank well, thereby increasing the equipment cost and the operation cost.
Regarding positions for supplying the waste plastic in the method, the waste plastic is fed from the upper portion of the equipment when the pyrolysis equipment is of a vertical type (see FIG. 3 of Patent Document 2). When the pyrolysis equipment is of a horizontal type (see FIG. 5 of Patent Document 2), the waste plastic is fed from the upper portion of the most upstream device of the equipment.
The waste plastic is pyrolyzed while allowing the waste plastic and high-temperature sand to move from the upper portion to the lower portion or from the upstream to the downstream depending on the feeding positions. However, when fluidization gas is supplied in the method, the mixture of the waste plastic and the high-temperature sand is deteriorated, thereby not efficiently recovering the pyrolysis products from the waste plastic.
Patent Document 3 discloses a method of recovering gas or oil from waste plastic by means of pyrolysis by mixing the waste plastic with thermal storage mediums.
This method includes a step of forming a movable bed formed of thermal storage mediums in a reactor, a step of introducing waste plastic into the reactor and pyrolyzing the waste plastic by means of heat of the thermal storage mediums while moving the waste plastic along with the movable bed, a step of extracting and recovering volatile pyrolysis products of the waste plastic from the upper portion of the reactor, a step of introducing high-temperature steam into a lower layer of the movable bed in the reactor to serve as carrier gas for discharging the pyrolysis products of the waste plastic out of the system and re-pyrolyzing the waste plastic by means of thermal energy of the high-temperature steam, and a step of discharging the pyrolysis products, non-volatile unpyrolyzed materials, and the thermal storage mediums from the lower portion of the reactor.
However, in the system in which the thermal mediums are stacked in the pyrolysis tank, the flow in the pyrolysis tank is not sufficient and the temperature distribution or the waste plastic distribution in the pyrolysis tank is not uniform. Accordingly, the volume of the equipment cannot be efficiently used.
Since the fed waste plastic can be easily lumped, pyrolysis heat is not efficiently transmitted to the waste plastic, thereby reducing the throughput. When the lump of the waste plastic becomes greater, it clogs the inside of the equipment, thereby making stable operation difficult.
Patent Document 4 discloses a method of pyrolyzing solid waste by allowing a mixture layer of solid waste and solid thermal medium to flow by the use of an agitator in a pyrolytic furnace. However, in this method, since the mixture of the solid waste and the thermal medium is made to flow only by the use of the agitator, there is a problem that the flow is not sufficient and a long-term stable operation is not possible.
Patent Document 5 discloses a method of pyrolyzing thermoplastics by bringing the thermoplastics into contact with a fluidized bed of heated solid particles. In this method, since steam is used as fluidization gas in a pyrolysis step, there is a problem that the flow is not sufficient only by the use of the steam and a long-term stable operation is not possible. For example, when it is intended to pyrolyze methacryl resin including methyl methacrylate units, since the boiling points of methyl methacrylate and water are substantially equal to each other (100° C.), it is not substantially possible that only methyl methacrylate is in a liquid state and steam is in a gaseous state. In order to circulate steam for use, both methyl methacrylate and steam should be made to be liquids, methyl methacrylate should be separated from water, and then the water should be heated to generate steam, thereby complicating the processes. There is also a problem that the method cannot be used for resin having a boiling point higher than that of water.
Patent Document 6 discloses a method of pyrolyzing methacryl resin by bringing the methacryl resin into contact with a high-temperature thermal medium fluidized mechanically. In this method, since the inside of a pyrolysis tank is fluidized only by the use of an agitator without using any gas, there is a problem that the flow in the pyrolysis tank is not sufficient and a long-term stable operation is not possible. There is also a problem that the flow in the pyrolysis tank is completely stopped depending on the temperature condition or the resin condition. This method has a problem that the pyrolysis products of the resin cannot be smoothly discharged from the pyrolysis tank system and the recovered pyrolysis products decrease in quality.
Patent Document 1: Japanese Laid-Open Patent Application No. 51-42374
Patent Document 2: Japanese Laid-Open Patent Application No. 9-235563
Patent Document 3: Japanese Laid-Open Patent Application No. 6-228569
Patent Document 4: Japanese Laid-Open Patent Application No. 54-83002
Patent Document 5: Japanese Laid-Open Patent Application No. 59-111815
Patent Document 6: Published Japanese Translation No. 2002-526466 of PCT